1. Field of the Invention
The invention relates to a process for surface treatment of workpieces by glow discharge by providing voltage pulses for igniting and maintaining a glow discharge between a workpiece disposed in a receptacle and a counterelectrode.
2. Prior Art
It has been known to treat workpiece surfaces by glow discharge, thus exposing, for instance, metallic workpieces to surface transformation by nitration, etc. It is also possible to use glow discharge for coating, tempering, annealing, etc. Usually, the workpiece is used as the cathode and the walls of a vacuum vessel surrounding the workpiece as a counterelectrode, these electrodes being connected to a voltage source of some 100 to 1000 volts during the glow discharge, which extends over the total workpiece surface to be treated. The so-called active species which result in the desired surface modification are created thereby. The glow discharge is normally fed by direct current.
On the one hand, the voltage for glow discharge must be high enough to cover the total workpiece surface to be treated (complete glowing), while it must not be so high as to cause an arc discharge, as this would involve the risk of damage to the workpiece. If direct current voltage is continuously supplied for the glow discharge, it will be difficult to maintain the glow conditions. As a rule, use is made of a rapid disconnecting device which, in case of an arc discharge, will cause a short circuit to protect the workpiece accordingly. If direct current is used to feed the discharge, the temperature existing at the workpiece is particularly dependent upon voltage, pressure and gas type. This is a basic disadvantage because pressure and temperature, above all, should be decoupled for reasons relating to the process technique. On the other hand, workpiece temperature has an influence on the transition from glow discharge to arc discharge.
It has been known to feed the glow discharge with square-topped pulses having a variable pulse duty factor in order to decouple the process parameters and avoid the risk of arc discharges. If the pulse intervals are too long, reignition problems will occur, the pulse voltage sufficient to maintain an ignited glow discharge being sometimes not capable of causing the repeated ignition. Moreover, in the known processes, switching losses are high because the connecting and disconnecting operations are taking place during current flow. Therefore, the pulse processes have not been successful to date.